1. Field of the Invention
This invention relates to adhesive substratum compositions (e.g. subbing compositions) for coating hydrophobic, macromolecular film bases. More particularly, this invention relates to subbing compositions which can be applied to said film base after the base has been biaxially stretched. Still more particularly, this invention relates to subbing systems having greatly improved anchorage to subsequently applied hydrophilic layers when applied after stretching.
2. Description of the Prior Art
During the manufacture of coated films (e.g. particularly photographic films) it is conventional to coat the hydrophobic, macromolecular elements used for the film support with a subbing composition which will provide anchorage for subsequently applied hydrophilic coatings (e.g. gelatin and gelatino silver halide emulsions). The prior art describes numerous elements which are useful as this primary subbing composition. Alles et al., U.S. Pat. Nos. 2,627,088 and 2,698,240 and Swindells, U.S. Pat. No. 2,698,235 disclose products and processes which embody the use of vinylidene chloride copolymers and particularly a terpolymer of vinylidene chloride with acrylic esters and itaconic acids which are eminently suitable as the primary subbing layer on said hydrophobic film supports.
Cohen, U.S. Pat. No. 3,460,944 filed Mar. 18, 1965 describes a method of further improving the anchorage of the subbing layers described above by mixing the primary composition terpolymer with an alkyl acrylate or alkyl methacrylate polymer. Further improvements in anchorage and a reduction in repellency spots and masking of scratches was obtained by polymerizing the alkyl acrylate or alkyl methacrylate directly in the aqueous dispersion of the terpolymer as taught, for example by Rawlins in U.S. Pat. Nos. 3,443,950, May 13, 1969 and 3,567,452, Mar. 2, 1971. These subbing compositions are conventionally applied from an aqueous dispersion on to cast polyethylene terephthalate films prior to a biaxial stretching of the film which results in biaxial orientation. During the continuous processing of a web, it is not uncommon for tears to occur for various reasons, such as imperfections in the cast film, misalignment of equipment, failure to maintain proper operating conditions, etc. These tears are most likely to occur during the biaxial stretching and heat setting step after the polymer is cast into film and coated with the subbing composition. Because of the presence of the subbing composition adhered thereto, it is difficult to recycle scrap film made in this manner due to arduous procedures necessary to remove the dried sub-layer. Numerous attempts have been made to coat these subbing compositions on the film support after the biaxial stretching and heat setting step but in all cases, the adherence of a subsequently applied water permeable colloid layer, e.g. gelatin is greatly lessened. D'Cruz in U.S. Pat. No. 3,545,972, Dec. 8, 1970 describes a subbing system which can be applied to a hydrophobic film base after biaxial stretching. However, this system requires that the film base be additionally treated with electrical discharge, flame treatment, surface oxidation, UV light exposure and the like.